The present invention relates to a liquid crystal device utilizing a self-alignment characteristic and electroconductivity of a liquid crystal, more specifically a liquid crystal device used in electronic devices, such as an organic electroluminescence device including a carrier-transporting layer comprising a liquid crystal and transistors or diodes utilizing the electroconductivity of a liquid crystal.
Extensive study is being made on the applied uses of an organic electroluminescence device (hereinafter sometimes called an xe2x80x9corganic EL devicexe2x80x9d) as a high speed response and high efficiency luminescence device. Such an organic EL device basically has a sectional structure as shown in FIG. 1A or 1B including a metal electrode 1, a luminescence layer 2, a hole-transporting layer 3, a transparent electrode 4, a transparent substrate 5, and an electron-transporting layer 6, e.g., as disclosed in Macromol. Symp., vol. 125, pp. 1-48 (1997).
As shown in FIGS. 1A and 1B, the organic EL device generally has a laminated structure comprising a transparent substrate 5, a transparent electrode 4 and a metal electrode 1, between which one or more organic layers are disposed. The structure of FIG. 1A includes a luminescence layer 2 and a hole-transporting layer 3 as the organic layers. The transparent electrode 4 may comprise a transparent conductor having a large work function, such as ITO (indium tin oxide), so as to exhibit a good hole-injection characteristic from the transparent electrode 4 to the hole-transporting layer 3. The metal electrode 1 may comprise a metal material having a small work function, such as aluminum, magnesium or an alloy of these, so as to exhibit a good electron-injection characteristic to the organic layers. These electrodes may have a thickness of ca. 50-200 nm.
In the organic EL device of FIG. 1A, the luminescence layer 2 typically comprises an aluminum quinolinol complex derivative, representatively Alq3 (tris(8-quinolinolato)aluminum) represented by a formula shown below, having both electron-transportation and luminescence characteristics Further, the hole-transporting layer 3 may comprise an electron-donative material, e.g., a triphenyldiamine derivative, representatively xcex1-NPD (bis[N-(1-naphthyl)-N-phenyl]benzidine of a formula shown below. 
An organic EL device having a structure as described above shows a rectifying characteristic, and when a voltage is applied between the metal electrode 1 as a cathode and the transparent electrode 4 as an anode, electrons are injected from the metal electrode 1 into the luminescence layer 2 and holes are injected from the transparent electrode 4. The holes and electrons injected to the luminescence layer 2 are recombined in the luminescence layer 2 to form excitons, which cause luminescence. In this instance, the hole-transporting layer 3 functions as a layer for blocking electrons to provide a higher recombination efficiency at the luminescence layer/hole-transporting layer boundary, thereby providing an enhanced luminescence efficiency.
In the structure of FIG. 1B, an electron-transporting layer 6 is disposed between the metal electrode 1 and the luminescence layer 2 in the structure of FIG. 1A. According to this structure, the luminescence function is separated from the functions of both electron transportation and hole transportation, to provide a structure exhibiting more effective carrier blocking, thus realizing more efficient luminescence. The electron-transporting layer 6 may comprise, e.g., an oxadiazole derivative.
The organic layers (including the luminescence layer 2, hole-transporting layer 3 and electron-transporting layer 6 may have a thickness of 50-500 nm in total of the two or three layers.
In the above-mentioned organic EL devices, the luminescence performance is critically determined by the performance of injection of electrons and/or holes from the electrodes. The above-mentioned amorphous materials, such as Alq and xcex1-NPD, are not believed to provide sufficient carrier injection performances in view of the resultant electrode-organic layer boundaries.
Accordingly, liquid crystal materials have been expected to provide new electron-transporting layer and/or luminescence layer materials exhibiting high carrier injection performance and high mobility.
Liquid crystal materials exhibiting a high carrier-transporting performance may include liquid crystals having a discotic liquid crystal phase or a high-order smectic phase, i.e., discotic liquid crystals and smectic liquid crystals.
Examples of the discotic liquid crystals may include triphenylene-type liquid crystals having structures as shown below (as disclosed in Adv. Mater., vol. 8, No. 10 (1996)). The below-indicated LC Compounds 1-4 having side chains or substituents R of alkoxy groups xe2x80x94OC4H9, xe2x80x94OC5H11 and xe2x80x94OC6H13 and a thio-ether group xe2x80x94SC6H13 are known to exhibit a hole-transporting performance at a high carrier mobility (on the order of 10xe2x88x921-10xe2x88x923 cm/Vs). These compounds exhibit a discotic columnar phase, wherein disk-shaped liquid crystal molecules are aligned to form a columnar shape so that their triphenylene skeletons rich in xcfx80-electrons are mutually superposed, thus exhibiting a good hole-transporting characteristic via the triphenylene group. LC Compound 5 shown below was developed by our research group and, because of poly-fluorinated side chains, exhibits a discotic liquid crystal phase range shifted to a lower temperature side and a lower ionization potential than the corresponding non-fluorinated compound. LC Compound 7 has a dibenzopyrene skeleton and also exhibits a discotic columnar phase. 
LC Compound 1: R=SC6H13 
LC Compound 2: R=OC4H9 
LC Compound 3: R=OC5H11 
LC Compound 4: R=OC6H13 
LC Compound 5: R=OC4H8C2F5
LC Compound 6: L=OC5H11
LC Compound 7: R=OC5H11
Other discotic liquid crystals may include those having skeletons of phthalocyanine derivatives, naphthalocyanine derivatives, truxene derivatives, hexabenzocolonene derivatives and benzoquinone derivatives.
Representative smectic liquid crystal materials may include LC Compounds 8-11 shown below (as disclosed by Ohyou Butsuri, Appl. Phys., vol. 68, no. 1, p. 26 (1999)). 
LC Compound 8
LC Compound 9
LC Compound 10
LC Compound 11
LC Compound 8 shown above, classified as a phenylbenzothiazole derivative and showing smectic A phase, has a hole-transporting characteristic. LC Compound 9, classified as a phenylnaphthalene derivative, shows smectic A phase and smectic E phase and a higher mobility in the smectic E phase of a higher order than the smectic A phase. LC Compound 9 also exhibits bipolar carrier (i.e., hole and electron)-transporting characteristic. LC Compounds 8 and 9 both show a high mobility of 10xe2x88x923 cm/Vxc2x7s or higher.
Other liquid crystal compounds having a bar-shaped skeleton and showing a smectic liquid crystal phase may also be used.
Such a liquid crystal compound, when used in the electron- or hole-transporting layer 3 or 6 of the structures shown in FIGS. 1A and 1B, is expected to provide a device of an improved performance at a good productivity.
The characteristics of a carrier-transporting liquid crystal material may be summarized as (i) a high carrier mobility due to self-alignment characteristic in the bulk state and (ii) a high carrier-injection characteristic due to a xcfx80-electron conjugated plane aligned parallel to the electrode boundary, which are not possessed by the conventional materials.
We have further studied whether more effective luminescence can be attained by generating carriers in an organic layer in addition to carrier injection from the electrodes. Hitherto, several studies have been reported regarding the doping of a carrier-transporting material with an electron-accepting or electron-donating compound in an organic layer, e.g., in (1) Yamamoto, et al., Appl. Phys. Lett., vol. 72, no. 17, p. 2147 (1998) and (2) Kido, et al., Appl. Phys. Lett., vol. 73, no. 20, p. 2866 (1998).
The above reference (1) reports that a polymeric material constituting a hole-transporting layer is doped with 20 mol. % of a salt containing SbCl6xe2x88x92 so as to generate holes and increase the carrier density in the hole-transporting polymeric material, thereby succeeding in high luminance emission. The above reference (2) reports an improvement in electron injection performance by doping an electron-transporting layer with Li metal.
Doping of liquid crystals has been reported, e.g., in (3) Boden, et al., J. Am. Chem. Soc., vol. 116, no. 23, p. 10808 (1994) and (4) J. Mater. Sci.: Materials in Electronics, vol. 5, p. 83 (1994).
The above reference (3) reports that a discotic liquid crystal material having a tricycloquinazoline is doped with 6 mol. % of potassium to provide an n-type semiconduction for transporting electrons as a principal carrier. The reference (4) reports that a discotic liquid crystal material having a triphenylene skeleton is doped with AlCl3 to provide a p-type semiconductor for transporting holes as a principal carrier.
In the case where an electronic device including a layer of liquid crystal material doped with an inorganic material is supplied with an external electric field, not only electronic carriers (holes or electrons) but also ionic (anionic or cationic) dopants are moved in the liquid crystal layer under the external electric field, thus causing an ionic current. The ionic current is caused by the movement of the dopants per se and thus results in a poor reversibility of current characteristic, thus leaving problems in not only initial performance but also long-term performance. Particularly, in the case of a liquid crystal material having a liquid characteristic, the ionic current problem is liable to be more serious than in the case of amorphous or polymeric materials. The above-mentioned references (3) and (4) were both directed to a device including a liquid crystal layer alone of which the voltage-current characteristic was measured as a basic property, so that a serious problem was not believed to be encountered even if an ionic current and an electronic current were both present.
In view of the above-mentioned problems, a principal object of the present invention is to obtain a liquid crystal composition by utilizing an effective doping technique stably applicable to a liquid crystal and provide a liquid crystal device capable of constituting an electronic device showing good performances by using the liquid crystal composition.
According to the present invention, there is provided a liquid crystal device comprising a pair of electrodes and a plurality of layers disposed between the electrodes and including at least one liquid crystal layer, which comprises a liquid crystal composition having an electronic carrier-transporting function and comprising at least two compounds including at least one electron-donating compound or electron-accepting compound having a xcfx80-electron conjugated structure.
In the liquid crystal device according to the present invention, the liquid crystal layer included therein and having an electronic carrier-transporting function contains an electron-donating compound or an electron-accepting compound having a xcfx80-electron conjugated structure, whereby carriers can be stably formed by mutual interaction with another compound therein. As a result, the liquid crystal is effectively and stably doped to exhibit a good electronic current characteristic. According to the present invention, it becomes possible to realize an electronic device, such as a semiconductor device or a luminescence device, exhibiting remarkably improved performances.